1. Field of the Invention
The present invention relates to a method for forming a photonic band-gap structure (PBG structure) on a substrate and a device having a photonic band-gap structure that is fabricated according to such a method for application in, for example, microwave and/or millimeter wave technology, that is, in the high frequency field.
Although applicable to any passive device, the present invention and the problems it is based on are described hereinbelow in regard to microwave and millimeter wave filters and electromagnetic hollow cavities, that is, micro cavities.
2. Description of the Background Art
There is generally much interest in transmitting and guiding electromagnetic waves for a wide range of application, for example, in the fields of cordless telecommunication, motor vehicles, and aircraft radar equipment.
Intensive research is currently being done with photonic band-gap structures (PBG structures) in the field of optical applications as well as applications in the microwave and millimeter frequency field. An electromagnetic band gap (EBG), which is also referred to as a photonic band-gap crystal (PBC) or as an electromagnetic crystal structure (ECS), includes a periodic array of inclusions in a material, which form a stop-band for defined frequency ranges. Photonic crystals, or PBG structures, are processed materials with periodic spatial variations of the dielectric constant. Based on a Bragg reflection, electromagnetic waves having defined frequency ranges cannot pass through the photonic crystal and, therefore, no resonant modes can occur. These frequency intervals are referred to as photonic band gaps. The energy does not spread in predefined directions within this stop band. In other words, photonic crystals are artificial crystal structures that have an effect on electromagnetic waves that is similar to the effect a semiconductor crystal has on electronic waves. An EBG defect is, as described above, an interference in the EBG lattice structure, whereby the defect may be realized by inclusion or absence of an atom or a molecule, in an otherwise periodic lattice. A defect such as this creates a narrow pass-band frequency range within the larger stop-band frequencies. The quality of the defect defines the width of this pass-band range. The field of periodic electromagnetic materials is currently one of the fastest-developing areas in electromagnetic technology. Periodic structures, for example, photonic crystals, can control the spreading of electromagnetic waves in ways that were unknown until recently.
Application possibilities for such PBG structures are microwave appliances, antennae, optical lasers, filters, resonators etc. For example, the quality factor of a cavity resonator is determined on a dielectric basis for a resonant mode by two loss mechanisms, namely, dielectric losses due to the dielectric materials that are used and metallic losses due to surface currents in the metallizations.
It is thus generally desirable to realize an integrated planar EBG, that is, a PBG structure for high frequency applications, that is, applications in the microwave and millimeter wave fields with minimal power dissipation.
A conventional approach exists, wherein a patterned metallic-dielectric and dielectric EBG structure for forming a high-quality resonator for microwave applications is provided. Conventional methods for producing such a resonator are costly and the components of the resulting structures are big in size and are not compatible with silicon-based technologies for the fabrication of integrated semiconductor circuits. However, silicon-based technologies have proven to be particularly beneficial so that future structures should be silicon-compatible ones.
In a further conventional approach, PBG structures are used for producing filters by utilizing patterned coplanar metallizations, or microstrip metallizations. The reduction of the filter size in such structures results in greater LC constants.
The disadvantage of this approach, however, has proven to be the fact that with these methods, components are constructed that can only be used in filter applications and not, for example, in micro-cavity applications for resonators. Furthermore, the structure of devices such as these requires several periods of artificial cells of electromagnetic crystals equal to half the wavelength of the signal. This results in big dimensions of the produced devices.